1. Technical Field
The present disclosure relates to the field of optical probes for use in near-field scanning optical microscopy (NSOM), also commonly referred to as scanning near-field optical microscopy (SNOM).
2. Description of the Prior Art
Near-field optical microscopy (NSOM) forms an image of a specimen by scanning an aperture over specimen or object. Utilizing a sub-wavelength aperture, which is an aperture smaller than the wavelength of light used to form the image, allows spatial resolution surpassing the diffraction limit. The aperture, an important determinative component of NSOM, has usually been formed on the end of an optical fiber which had been tapered by mechanical pulling or by chemical etching and then coated with aluminum as described in “Near-Field Nano/Atom Optics and Technology,” edited by M. Ohtsu, and published by Springer-Verlag, 2002. In the prior art, mechanical pulling, as described by G. A. Valaskovic in “Parameter control, characterization, and optimization in the fabrication of optical fiber near-field probes,” Appl. Opt., vol. 34, pages 1215-1228 (1995), involves first heating a small portion the fiber with, for example, a CO2 laser beam and then rapidly pulling along the axis of the fiber causing the heated region to taper and eventually separate so as to form one or two fibers with tapered ends. This approach suffers from very high attenuation of the optical power traveling within the tip, which limits the utility of NSOM in experiments such as near-field Raman Spectroscopy, single molecule fluorescence, etc.
Chemical etching fabrication methods require one or more mechanisms to cause the cladding and core to etch at different rates. Shuji Mononobe and Motoichi Ohtsu describe such methods in their publication, “Fabrication of it Pencil-Shaped Fiber Probe for Near-Field Optics by Selective Chemical Etching,” Journal of Lightwave Technology, vol. 14, no. 10, October 1996. The mechanisms may be grouped into two categories: meniscus etching and selective etching. As described in Mononobe et al, meniscus etching uses an oil layer floating on the top of the etching mixture with the interface forming a meniscus around the fiber. The height of the meniscus drops as the fiber cladding diameter decreases during the etching process. P. Hoffmann, B. Dutoit, and R. P. Salath describe tips formed using this mechanism in their publication entitled “Comparison of mechanically drawn and protection layer chemically etched optical fiber tips,” Ultramicroscopy, vol. 61, pages 165-170 (1995). The surfaces of these tips produced by this method frequently show scalping and other defects caused of the unstable balance between gravity, surface tension and van der Waals forces involved in chemical wetting of the fiber surface. Further, the apex of the resulting tips may not be well centered within the core again because of this instability.
Selective etching relies on the fact that fibers with cores doped, for example, with germanium, placed into an etching solution of, for example, a buffered hydrofluoric acid solution with a volume ratio of [ammonium fluoride solution (40 wt %)]: [hydrofluoric acid solution (50 wt %)]: [water]=[X]:[1]:[1] respectively where X is the variable, the core etches faster than the cladding for X<1.7 and the cladding etches faster than the core for X>1.7 up to about X=30. The simplest implementation of selective etching places an optical fiber directly into an etching solution with X=30. Over time, the cladding etches back and away from the core slowly exposing the sides of the core to the etching solution. The result is a conical tip protruding from the end of the fiber. The remaining cladding forms a comparatively large shoulder which interferes with light propagation in reflective NSOM configurations. The advantage of selective etching is the automatic centering of the tip with respect to the core and the smooth surface of the tip.
These two mechanisms may be applied in various ways to produce an NSOM tip with a both a tapered core tip and cladding. Mononobe et al. produced tips using a four step process. The obvious disadvantages of a multi-step process are additional processing costs and the increased opportunity for damage to the tips in process.
After etching the tapered tips are usually coated with aluminum which retains the traveling radiation within the fiber. The details of metalizing NSOM tips are well described in the prior art, for example, by Saeed Pilevar, Klaus Edinger, Walid Atia, Igor Smolyaninov, and Christopher Davis in their publication “Focused ion-beam fabrication of fiber probes with well-defined apertures for use in near-field scanning optical microscopy,” Appl. Phys. Lett., vol. 72, page 3133 (1998) and by T. Saiki, S. Mononobe, M. Ohtsu, N. Saito, and J. Kusano in their paper entitled “Tailoring a high-transmission fiber probe for photon scanning tunneling microscope,” Appl. Phys. Lett., vol. 68, page 19, (1996). After the coating process, it may be necessary to remove some aluminum from the very tip of the coated region. This may be performed using a focused ion beam also as described by Saeed Pilevar et al.
Another critical consideration of NSOM imaging is the polarization of the illumination, which, among its other effects, is one of the factors which influence the optical contrast as described by E. B. McDaniel and J. W. P. Hsu in “Nanometer scale optical studies of twin domains and defects in lanthanum aluminate crystals,” J. Appl. Phys., vol. 80, pages 1085-1093 (1996); by E. Betzig, J. K. Trautman, J. S. Weiner, T. D. Harris, and R. Wolfe in “Polarization contrast in near-field scanning optical microscopy,” Appl. Opt., vol. 31, pages 4563-4568 (1992); and by J. A. Cline and M. Isaacson in “Probe-sample interactions in reflection near-field scanning optical microscopy,” Appl. Opt., vol. 3, pages 4869-4876 (1995).
In a conventional circular core optical fiber, it is very difficult to control the polarization, which changes in response to stress and temperature induced geometrical non-uniformities in the core-clad geometry. T. Mitsui and T. Sekiguchi describe tips produced from stress-induced birefringent fibers mechanical pulling in their publication, “Observation of polarization property in near-field optical imaging by a polarization-maintaining fiber probe,” Journal of Electron Microscopy, vol. 53, no. 2, pages 209-215 (2004).
Additional results may be found in T. Mitsui, “Development of a polarization-preserving optical-fiber probe for near-field scanning optical microscopy and the influences of bending and squeezing on the polarization properties,” Rev. Sci. Instrum., vol. 76, pages 043703-1-043703-6 (2005). The resulting tips have very low throughput, which limits the utility of NSOM in experiments such as Near-field Raman Spectroscopy, single molecule fluorescence, etc.